I raced my '65 GT fastback for several years basically stock. For upgrades, I looked to what was used in back in the day. For example, original export brace which is stiffer than repro, Monte Carlo bar, my car has a nine inch rear from a K code with upgraded rear drums 10"x2-1/2" from a station wagon, larger 24" radiator from a full size Ford or '69 mustang, etc, etc. Make sure you do a full on tech for your car prior, complete brake fluid flush and new pads. You may want to get a spare set of wheels and tires with a wider footprint and alignment. Good luck and let the rear end hang out there via the throttle!

Export brace - either one piece, or install a firewall connectore to join up a two piece brace..

Frame to shock tower gussets - the ones outside the engine bay, under the upper control arm mounting point.

Straight Monte Carlo bar.

Reverse the spring perches - pushes the base of the coil spring out-board about 3/4 inch, and helps roll stability and control under heavy driving - and can add stress on the lower shock mount, so be sure to have quality shocks and perch bolts.

 

I don't know about all that stuff, but I do know that the last time I did an autocross, my 68 did not like braking hard then immediately turning hard left while entering a circle. The engine died at exactly the same spot twice in a row. Something about the Edelbrock 1406 and the stock floats installed. I was told that there were special floats made for that carb for autocross. Don't know about that as I decided that go fast straight ahead was more to it's liking...

You mentioned the Shelby drop and tires, those are the biggest for a close to stock early Mustang. Now, depending on the tires and the front end settings, you may not need much else. Some tires like a lot of negative camber to work well, so you may want to look at front alignment to get some more grip out of your tires. Depending on your driving style, a 1.125 to 1.250 front sway bar should work well. A small rear bar should be all you need, maybe .50 to .6250. Double adjustable shock can be a help if you learn to adjust them. Remember cars need very little rebound damping so set the rebound very low. My car is fastest when it seems to be bouncy in a straight line. Too much rebound damping lifts the tire off of the track---Not good! BUT Remember to add one thing at a time and try it, so you get a feel for each modification. 

1" sway bar and both Monte Carlo bar and 1 piece export brace made a HUGE difference in mine. This along with the Shelby drop may be all you need to have fun. If not....add the other suggestions later.
6s6

  Let's see if I can remember all the numbers...
720 front coils KYB with Shelby drop
-1.4 camber
1.25" front sway w/poly links
Kuhmo XS (DOT 200 class tire)
Trans Am disks on 1970 spindles
Export and Monte Carlo
Rear mount battery
Remove/unscrew inside rear view mirror (to easily see around corners)
Left foot brake hard if staying in same gear into/thru/and out of the corner

Last edited by Prof (4/27/2015 7:29 PM)

Stopping is definitely necessary, and you need to not lose power anywhere due to carb issues.  The thing to remember with suspensions if you can either jack up the spring rates and flex the car into the pavement, or you can stiffen the chassis and run more comfortable spring rates.  720s are pretty brutal on the strret.  Sway bars can help, but should really be used primarily for tuning the over/under streer characteristcs of a car.  Your best setup is going to be mild understeer which can be balanced with throttle induced oversteer. 

What will improve your times more than anything is to improve your driving.  If you've never done it, a track course like Skip Barber, ect, is highly recommended.  I've always been a casual racer, but I've taken a few courses over the years and I've spanked guys out of my league car wise at events because I could drive and they couldn't.  A Porsche 911 with a roll cage and a Z06 Vette all turned slower lap times than I did in an '02 WRX wagon with only upgraded rotors and pads.  Of course I also ran consistent 14.20s with that car in the 1/4 when the best magazine times I ever saw were 14.50s.  Practice does tend to make perfect.